Deterioration of concrete is known to occur due to chemical reactions between certain hydraulic cement constituents and those constituents present in the aggregate of a concrete mix. Certain materials are known to be reactive with the alkalies in cements. These materials include various forms of silica, such as opal, chalcedony, tridymite and cristobalite. For example, it is known that when hydrated silica which is inherent in aggregate comes into contact with sodium or potassium at high pH (the high pH being caused by hydroxyl ions present from alkaline compounds of cement), alkali silicate is formed. Alkali may also be derived from external sources such as deicing salts and sea water, as well as from internal sources such as admixtures, fly ash and alkali-bearing aggregates. Alkali silicates, which are the product of this "alkali-silica reaction", form a water swellable expansive gel deposit within and on the exterior surface of the aggregate particles contained in concrete. Under moist conditions, the swelling of this gel can eventually produce sufficient pressure to induce deleterious cracking and expansion of the concrete mass.
As a means to combat the "alkali-silica reaction", Japanese Kokai Patent Sho 62-278151 proposes adding Li.sub.2 O to the cement during the course of preparing the concrete. Specifically, 0.01 to 5% by weight of Li.sub.2 O is added to hydraulic cement that contains aggregates that can cause deleterious reactions. Disclosed sources of lithium include lithia mica, leafy feldspar, lithia pyroxene (i.e. the naturally occurring spodumene mineral, which is alpha-spodumene) containing 4 to 9.5% Li.sub.2 O, and petalite. The reference teaches that the lithium source is preferably added as a powder to form a uniform mixture with the cement.
U.S. Pat. No. 3,331,695 discloses mortar (cement, sand and water) and paste (cement and water) compositions which can achieve an accelerated hardening rate by adding 0.1 to 20% spodumene (LiAlSi.sub.2 O.sub.6), based on the dry weight of the cement binder, to the cement of such cement compositions. The cement compositions of this '695 patent did not contain aggregate having the type of silica capable of entering into the deleterious alkali-silica reaction to exhibit cracking and expansion.
It is therefore an object of the present invention to provide a cement admixture capable of inhibiting alkali-silica reaction. It is a further object of the present invention to provide an improved concrete composition capable of inhibiting cracking and expansion as a result of alkali-silica reactions.